Osmolal effects on vasopressin secretion in the streptozotocin-diabetic rat

To determine the relationships between serum levels of arginine vasopressin (AVP) and serum osmolality and sodium in diabetic rats, we measured these variables in streptozotocin-diabetic and normal animals treated with water, isotonic saline, or hypertonic saline. Serum osmolality was higher and sodium lower in untreated diabetics than in controls; these variables increased in both groups after hypertonic saline. Serum AVP levels (measured by radioimmunoassay in Amberlite-extracted serum) were 2.3 +/- 0.5 and 9.8 +/- 1.7 pg/ml in control and diabetic rats, respectively, injected with isotonic saline. AFter injection of hypertonic saline, serum AVP levels rose to 14.5 +/- 2.3 pg/ml in controls and 18.7 +/- 1.7 pg/ml in diabetics. Water injection decreased serum AVP in diabetics (as in normals), but only to 5.8 +/- 1.0 pg/ml. To assess indirectly whether the chronically high levels of AVP in serum had an impact on kidney function, diabetic rats were studied after passive immunization with an anti-AVP serum. This treatment increased urine flow and decreased urine osmolality in dehydrated diabetic rats. Taken together, these data affirm in diabetic rats, as in humans, the occurrence of 1) elevated steady-state levels of AVP in serum; 2) abnormal sensitivity of AVP secretion to changes in serum sodium and osmolality; and 3) an apparently intact end-organ responsiveness to AVP.